FIG. 17 is a configuration diagram showing a known heating power supply apparatus according to a patent document 1. In the drawing, reference numeral 1 denotes a four-winding transformer; 2, 3, 4, and 5 denote thyristor rectifiers connected to secondary coils of the four-winding transformer 1; and 14, 15, 16, and 17 denote negative resistance loads connected to each of the thyristor rectifiers 2 to 5. Reference numerals 6, 7, 8, 9, 10, 11, 12, and 13 denote thyristor switches as semiconductor switches connected in circuits (lines), for connecting the thyristor rectifiers 2 to 5 to the negative resistance loads 14 to 17. The loads 14 to 17 have negative resistance characteristics; and therefore, a small current at high voltage is required at initial energization, and a large current at low voltage is required at final stage energization.
As shown in FIG. 17, the known heating power supply apparatus is provided with eight circuits of the thyristor switches for switching modes on the load sides of four circuits of the thyristor rectifiers which convert alternative current to direct current in order to realize an improvement in power factor with respect to the negative resistance loads and a reduction in installed capacity. In the process when the resistive load is reduced, energization is performed by switching conductive/nonconductive states of the thyristor rectifiers and the thyristor switches into three modes: in an initial stage, “individual energization to four loads”; next, “energization to two loads in series”; and in a final stage, “energization to four loads in series.”
Particularly, as shown in FIG. 18, in a large current region at the final stage, energization is performed via the thyristor rectifier 2 and two thyristor switches 9 and 10, and the thyristor rectifier 4 and three thyristor switches 6, 9, and 13; and therefore, electric power loss of the thyristor switches is large. In addition, in the drawing, a dashed line represents a current direction; a black (painted out) thyristor of the thyristors represents an energized state, and a white (not painted) thyristor represents a de-energized state.
In the known heating power supply apparatus, the thyristor switches are provided on the output sides of the thyristor rectifiers to switch between series and parallel connection; and therefore, there is a problem that there causes an increase in electric power loss.
Patent document 1: Japanese Unexamined Patent Publication No. 62-1017